This invention relates to a color transforming method which intends to achieve visually faithful or preferred color reproduction of color images. More particularly, the invention relates to a color transforming method by which input digital image data are converted to the image signals required for the original image in an input (color space) system to be reproduced faithfully in an output (color reproduction) system having a different color gamut (color space) such as a different dynamic density range than in the input (color space) system, as well as a color transforming method for achieving transformation to image signals that are required to ensure that the important colors are reproduced preferably, that is, in a visually preferred lightness level, whether the input color space is the same as the output color space or not. More specifically, the invention relates to a color transforming method by which image signals read with a scanner or the like from transmission or reflection original hardcopy images obtained by photographing a subject on reversal films or negative films, or image signals obtained by photographing a subject directly with a solid-state imaging device such as a CCD, or image signals of an image displayed on a TV monitor are converted to the digital signals that are required for creating reproduced reflection hardcopy images visually faithful to the transmission original hardcopy images, the subject, the monitor and the like, or reproduced reflection hardcopy images on which the important colors are reproduced in a visually preferred lightness level, or for displaying reproduced softcopy images on the monitor or the like which are visually faithful to the transmission originals, reflection original, the subject and the like, or reproduced softcopy images on the monitor or the like in which the important colors are reproduced in a visually preferred lightness level.
In recent years, there is an increasing use of an image processing system which involves the reading of an exposed film with a scanner and subsequent conversion to digital image signals (the system is hereunder referred to as a "hybrid system") and a digital image processing system in which a subject is photographed with a digital camera or the like to obtain digital image signals directly. The digital and hybrid systems provide more flexibility in image processing than the analog system but, on the other hand, they suffer from increased costs. Therefore, the success of the hybrid and digital systems depends on whether the improvement in image quality justifies the increased cost.
The hybrid system uses the same input original as in the analog system, so in order to achieve an improvement in image quality, the image processing procedure has to be reviewed in terms of zero base. This is also true with the digital system. The image processing procedure can generally be classified into three stages of setup, range compression (extension) and design. These three elements of the image processing system have their own counterparts in the visual perception as follows: setup can be compared to luminance adaptation and chromatic adaptation, range compression (extension) is similarly named in the visual perception, and design modifications in the image processing system can be compared to retrieving colors from memory and rendering preferred colors.
Among the three elements of the image processing system, setup and design have been studied extensively to provide several important findings. On the other hand, only few findings have so far been attained in the study of range compression (extension). If studied at all, the range compression is in many cases discussed in terms of the relationship between the original scene and a hard copy but if the input range is sufficiently wider than the output range, the input need not be the original scene. In addition, from a practical viewpoint, the original scene is generally difficult to measure. For these reasons, in the following discussion of the prior art of range compression and its problems, we assume that a reversal film is the original which is range compressed to a photographic paper (hereunder "paper"). In order to circumvent the problem with setup, we also assume that the input original is limited to what has been exposed appropriately and in order to get around the problem with design, it is assumed that the paper should reproduce an image which, as perceived by the eye, is faithful to the original on the reversal film.
Speaking of the dynamic range, the paper is less flexible than the reversal film and the former has typically a density range of about 2.0 whereas the latter has a density range of about 3.0. Therefore, if the reversal film is simply output, the highlights and shadows become "flat" (lose contrast) to impair the image quality considerably. To deal with this problem, range compression is required. However, if range compression merely involves rendering the input original "less contrasty", the output picture is quite poor in aesthetic appeal on account of the deteriorated contrast and chroma. Thus, the reproduction of highlights and shadows and that of contrast and chroma are tradeoffs.
To deal with this situation, two techniques are currently employed in photography, printings and other areas of producing hardcopy images; one technique involves rendering the input image less contrasty and restoring the chroma by the "interlayer effect" or color correction, and the second technique commonly called "dodging" involves printing with the shadows in the exposed area being covered with a mask. However, these techniques have their own limitations. In the former technique, the hue fidelity deteriorates as the chroma improves and the skin color acquires a red tinge. In other words, the reproduction of chroma and that of hue are tradeoffs. Dodging which essentially involves a two-dimensional arithmetic operation is low in operating efficiency and is not cost-effective. Automatic dodging machines have recently been commercialized but the problem of high calculation loads is still incumbent.
In color science, range compression (extension) is a subject which is actively studied as part of gamut mapping on CIELab. Most of the studies so far made depend on the combined use of compression and clipping but the problem is that the timing of determining which method to use depends on the graphics pattern.
Fidelity as perceived by the eye is also required by TV monitors, video projectors and other machines that produce softcopy images; when subjects photographed with digital cameras, video cameras, etc. or images on transmission and reflection original hardcopy images as read with scanners, etc. are to be displayed as reproduced softcopy images on TV monitors, video projectors, etc. or when original softcopy image displayed on TV monitors, video projectors, etc. are to be replicated on reflection reproduced hardcopy images, it is required to reproduce output softcopy/hardcopy images that are faithful to the input hardcopy/softcopy as perceived with the eye. However, color gamut such as the dynamic density range do not necessarily coincide between the input and output spaces and the various problems described in the preceding paragraphs exist.
In both the hybrid and digital systems, the color transformation process for preparing reflection prints consists of gamma increasing and color correction steps. In the gamma increasing step, the density contrast of a reflection print is generally recommended to be higher than that of the subject by a factor of 1.8 (if the reflection print is to be prepared from the density data of a color negative film, the value should be increased to 2.5 (.congruent.1.8/0.7) in consideration of the fact that the characteristic curve of the color negative film has a gamma (.gamma.) of 0.7). The color correction step is often performed by the matrix operation using for example a 3.times.3 or 3.times.9 color correcting matrix.
The color transforming process is commonly adapted to be performed in the order of gamma increasing and color correction (see FIG. 9a) but the order may be reversed such that color correction is performed first (FIG. 9b).
The process designs shown in FIGS. 9a and 9b generally yield different results; however, if the gamma increasing is a linear process while the color correction is expressed by a 3.times.3 matrix, the two operations yield the same result as the following equation shows: ##EQU1##
In the color reproducing process, the colors of skin (face), green (grass) and sky (blue) are called "important colors" and often require selective processes for their reproduction. As for the reproduction of lightness, it is generally recommended that the skin color be finished "light" (faint) whereas the blue sky color "dark" (deep).
When an image formed on a copy or a first image forming medium is to be replaced on a second image forming medium, complex color reproducing processes have to be performed in order to ensure the preferred color reproduction that appropriate color reproduction is compatible with the selective reproduction of the important colors, in particular the skin and sky colors, in a visually preferred lightness level. To this end, persons having ordinary skill have carried out gradation modification in the field of plate making and dodging or other processing in the field of photography. Both the cases required highly skilled practice. Thus, there has been a strong need for a method by which colors (important colors) can be simply and selectively reproduced in a visually preferred lightness level, while being properly reproduced in the replication of the image.